In nuclear power plants, metal tubes or pipes may be used as heat exchanger tubes of a steam generator and as heat exchanger tubes of a heat exchanger such as a feed water heater. Hereinafter, heat exchanger tubes for a steam generator and heat exchanger tubes for a heat exchanger which are for use in nuclear power plants are also collectively referred to as metal tubes or pipes for nuclear power plants. Such metal tubes or pipes for nuclear power plants are made from a stainless steel exhibiting high corrosion resistance in a chloride-containing environment, or a nickel-based alloy exhibiting high intergranular corrosion resistance in a high temperature water environment.
Typically, in the manufacture of metal tubes or pipes for nuclear power plants, after tubes or pipes have been formed into a predetermined size, they are subjected to a solution heat treatment (a first heat treatment), and further to a second heat treatment for finishing. The second heat treatment may be intended to relieve residual stress of the metal tube or pipe resulting from straightening and polishing processes that are performed after the solid solution treatment (the first heat treatment). Furthermore, the second heat treatment may be intended to enhance the intergranular corrosion resistance of the metal tube or pipe by allowing precipitation of Cr carbides along the grain boundaries in addition to relieving residual stress as described above.
When the second heat treatment is intended for precipitation of Cr carbides along the grain boundaries in addition to relief of residual stress, it is accomplished by soaking at a temperature between about 690° C. and about 740° C. for 2 hours or more, preferably for 5 hours or more. When the second heat treatment is intended only for relief of residual stress, it is accomplished by soaking at a temperature between about 690° C. and about 740° C. for 0.5 hours or more, preferably for 2 hours or more.
The second heat treatment may be carried out in a batch fashion in which a plurality of metal tubes or pipes laid down on a cart, a tray, or the like are accommodated in a furnace and subjected to a heat treatment.
FIG. 1 is a diagram showing a cart which is for use in a heat treatment furnace, and on which metal tubes or pipes are laid down. FIG. 1(a) is a front view thereof and FIG. 1(b) is a side view thereof. The cart 2 shown in FIG. 1 is composed of carriages 21 having wheels, support pillars 23 fixed to each carriage, and cross beams 22 mounted between each pair of support pillars 23. When a heat treatment is applied to a metal tube or pipe in a heat treatment furnace using a cart of the type shown in FIG. 1, it is performed in such a manner that a plurality of metal tubes or pipes 1 are laid down on the cross beams 22, and in this state, the cart is accommodated in the heat treatment furnace to subject the metal tubes or pipes to heating.
It is noted that the metal tubes or pipes 1 are not limited to straight tubes or pipes as shown in FIG. 1, and that bent tubes or pipes formed by a bending process may also be heat treated by being accommodated in a heat treatment furnace in a state in which they are laid down on the cross beams 22. Furthermore, the metal tubes or pipes 1 may be stacked in two tiers with metal tubes or pipes being laid down on metal tubes or pipes that have been laid down on the cross beams as shown in FIG. 1.
Metal tubes or pipes for nuclear power plants typically have a diameter of 15 to 22 mm, a wall thickness of 0.9 to 1.3 mm, and a length of 15,000 to 27,000 mm, and thus they have a long length. Because of this, in order to prevent deformation of the laid down metal tubes or pipes during heat treatment, which may result from flexure thereof, the cart 2 is provided with a plurality of cross beams 22 arranged along the longitudinal direction of the metal tubes or pipes. Furthermore, in order to arrange the cross beams over the entire length of the metal tubes or pipes, the cart 2 is constituted by a plurality of carriages 21 that are coupled together.
In the case of metal tubes or pipes for nuclear power plants, if they undergo bending due to deformation caused by flexure during the second heat treatment, they will interfere with tube support plates when they are installed as heat exchanger tubes in a steam generator, and may break during operation, which could result in a serious accident. Thus, for heat exchanger tubes for a steam generator of a nuclear power plant, it is typically required that the amount of bending be 0.50 to 1.00 mm or less per 1000 mm longitudinal length.
If the amount of bending exceeds the tolerance, it is necessary to eliminate the bending by straightening the metal tubes or pipes. If straightening is performed through a cold rolling mill or a straightener, residual stress, which has been relieved from the metal tube or pipe by the second heat treatment, will occur again. Because of this, the straightening operation is inevitably carried out in such a manner as local straightening by hand work. Thus, the efficiency in manufacturing metal tubes or pipes significantly deteriorates if bending due to flexure occurs in the second heat treatment.
In the meantime, during heat treatment, a metal tube or pipe undergoes thermal expansion in a longitudinal direction thereof by being heated to an elevated temperature. During the process, a cart or a tray that is used for laying down of the metal tube or pipe also thermally expands, but its coefficient of thermal expansion differs from that of the metal tube or pipe, and long-length metal tubes or pipes exhibit a larger amount of deformation. As a result, rubbing occurs between the metal tubes or pipes and the region of the cart or the tray where the metal tubes or pipes are laid down, which results in scraping-off of the outer surface of the metal tubes or pipes so that recesses are formed therein.
Scratches in the form of recesses can be initiation sites for corrosion of a metal tube or pipe and could result in a serious accident when metal tubes or pipes are used in a nuclear power plant. Therefore, in metal tubes or pipes for nuclear power plants, it is required that the depth and length of scratches are equal to or smaller than those of a standard limit sample. In general, standard limit samples of scratches are fabricated by artificially scraping the outer surface of the metal tubes or pipes to form scratches. The formed scratches are 0.01 to 0.1 mm in depth and 10 to 100 mm in length.
If the depth and length of the scratches formed in the outer surface of the metal tubes or pipes in the second heat treatment exceed those of the standard limit sample, it is necessary to polish the outer surface of the metal tubes or pipes to eliminate the scratches. If the polishing is mechanically performed, residual stress, which has been relieved from the metal tube or pipe by the second heat treatment, will occur again. Therefore the polishing must be carried out by hand work. Consequently, the efficiency in manufacturing metal tubes or pipes significantly deteriorates.
With regard to inhibition of such bending or scratches resulting from heat treatment, there are various conventional proposals as disclosed in Patent Literatures 1 to 5, for example. Patent Literature 1 proposes a stacking tray unit, including: support pillars and a plurality of trays attached thereto at predetermined spacings, wherein each of the trays is composed of a plate member having higher thermal conductivity in a length direction than in a thickness direction. It is stated that high-quality products can be produced by performing a heat treatment using such a stacking tray unit, on which workpieces to be treated are laid down, because temperature variations can be reduced in the heated workpieces.
With the use of the stacking tray unit to heat treat metal tubes or pipes, it is possible to inhibit bending of the resulting metal tubes or pipes because the trays provide support for the metal tubes or pipes in a longitudinal direction with reduced temperature variations. However, the stacking tray unit proposed in Patent Literature 1 has a problem in that contact of the metal tubes or pipes with the trays causes formation of scratches on their outer surfaces along a longitudinal direction.
Patent Literature 2 proposes a cushioning material in sheet form, including: high temperature fibers and inorganic fibers which are mixed with each other or disposed in layers, and integrated by needle punching. Patent Literature 3 proposes a heat resistant cushioning material, including: a plurality of stacked heat resistant sheet materials which are integrated using a heat resistant thread or a high temperature adhesive. If metal tubes or pipes are laid down on cross beams with either of these cushioning materials interposed therebetween, a great effect of preventing scratches is obtained. However, the gas that is produced by heating the cushioning material causes discoloration of the outer surface of the metal tubes or pipes.
Discoloration of a metal tube or pipe impairs the aesthetics thereof and decreases its commercial value. Moreover, when they are used as a metal tube or pipe for nuclear power plants, the discoloration can be an initiation point for corrosion and could result in a serious accident. Thus, the discoloration of the metal tube or pipe needs to be eliminated by manual polishing, and this leads to significant deterioration of the manufacturing efficiency for metal tubes or pipes.
Patent Literature 4 proposes a method for heat treating a metal tube or pipe, including: interposing a heat resistant fabric having a thickness of 0.1 to 1.2 mm between a metal tube or pipe and cross beams on which the metal tube or pipe is laid down. Patent Literature 4 also discloses that, in order to remove water remaining in the heat resistant fabric that can be a cause of discoloration, it is preferred that, in the heat treatment, the heating in the temperature range between 100° C. and 300° C. be performed for 30 minutes or more. Thus, the method for heat treating a metal tube or pipe proposed in Patent Literature 4 may lead to deterioration of the manufacturing efficiency for metal tubes or pipes.
Patent Literature 5 proposes a steel bar accommodation pallet, including: a grid-shaped member disposed at a location where the workpieces to be treated are laid down. It is stated that the grid-shaped member structurally has high stiffness and good durability, and therefore, with a minimum weight, it is capable of supporting the workpieces to be treated and minimizing hindrance to heat input to them. If an accommodation pallet including such a grid-shaped member is used to heat treat a metal tube or pipe that is laid down thereon, it is expected that bending will be inhibited. However, the increase in contact area between the metal tube or pipe and the grid-shaped member results in formation of scratches on the metal tube or pipe.